1. Technical Field
The present invention relates to a memory device for holding information by storing charges moved from a conduction region into a storage region and a method of manufacturing the memory device, and a method of manufacturing an integrated circuit and a semiconductor device in which memory device are integrated. More particularly, the invention relates to a memory device in which a storage region comprises a plurality of particulates (quantum dots), a method of manufacturing the memory device, and a method of manufacturing an integrated circuit and a semiconductor device.
2. Background Art
In a non-volatile memory typified by an EEPROM (Electric Erasable-Programmable Read Only Memory), a storage region is formed on a conduction region formed on the surface of a single crystal silicon substrate with an insulating film in between. Charges moved from the conduction region by tunneling the insulating film are stored into the storage region, thereby information is held in a non-volatile memory. Hitherto, the storage region is formed by using a two-dimensionally spread continuous semiconductor film. In a conventional non-volatile memory, in order to hold charges stored in the storage region for a long time, a thermal oxide film made of silicon having excellent insulating properties is used as an insulating film between the conduction region and the storage region. The thermal oxide film is prepared by heating a single crystal silicon substrate to 800 to 1000.degree. C. in an oxygen atmosphere. The silicon substrate is not deformed nor melt around this temperature. Since the thermal oxide film formed on the single crystal silicon substrate has excellent insulating properties, charges are not leaked from the two-dimensionally continued storage region and are stably held.
As described above, in the conventional memory device, in case of using the thermal oxide film as an insulating film between the conduction region and the storage region, the substrate has to be heated to a high temperature of 800 to 1000.degree. C. When the substrate is made of glass or plastic material instead of single crystal silicon, however, such high-temperature heat treatment cannot be performed. For example, the temperature at which a glass substrate is deformed is 500.degree. C. The temperature at which a plastic substrate is deformed is at most 200.degree. C. even when the substrate is made of a heat resistant material. When the substrate is made of glass or plastic, therefore, the insulating film between the conduction region and the storage region has to be prepared in low-temperature conditions of 500.degree. C. or lower.
When the oxide film is prepared at a low temperature of 500.degree. C. or lower, however, defects, or a number of structural holes (pin holes) occur in the oxide film. These pin holes let the charges stored in the two-dimensionally spread storage region to be leaked to the conduction region in a short time and cause a problem that information cannot be held for a long time.
On the other hand, although an oxide film such as a silicon oxide film (SiO.sub.2) on the silicon substrate is resistant to heat of high temperature, the conduction region on the oxide film is made of polysilicon and the surface of polysilicon is rough. Due to the roughness of the polysilicon, concentration of an electric field occurs in the insulating film provided on the polysilicon, which causes a problem that the charges in the two-dimensionally continued storage region are also leaked from the position where concentration of an electric field occurs.